Extracorporeal fluid treatment typically involves the removal of a body fluid from a patient, treatment of the fluid externally to the patient, and return of the treated fluid to the patient. Blood is one body fluid for which conventional extracorporeal techniques have been developed. Using these techniques, blood is treated to extract materials from the blood and/or add materials to augment the blood prior to return of the treated blood to the patient. More particularly, extracorporeal blood treatment may be accomplished by removing the blood from the patient in a continuous flow and introducing the blood into a chamber containing a filtration unit, wherein the blood is conducted past a semi-permeable membrane. The semi-permeable membrane selectively allows material in the blood to pass through the membrane for removal from the blood and/or allows material to pass through the semi-permeable membrane to the blood, to or from a fluid separately flowing past the semi-permeable membrane of the filtration unit. After passage of materials to and from the blood, the treated blood is discharged from the filtration device for return to the patient. The material which has been removed from the blood is separately discharged from the filtration unit.
One exemplary extracorporeal blood treatment is hemodialysis. In conventional hemodialysis treatment, treated water is provided to a hemodialysis machine and mixed therein with a predetermined amount of one or more solutes or concentrates to form a dialysate. The water is heated in the hemodialysis machine before and/or after addition of solutes and/or concentrates, typically to a body temperature of approximately 37.degree. C. This fresh dialysate is then conducted into a filtration device or dialyzer of the hemodialysis machine. Once in the dialyzer, the dialysate flows past a side of a semi-permeable membrane, typically in a counter-current direction to that of the blood from a patient flowing in the dialyzer on an opposite side of the membrane. Waste matter, typically organic molecular ions, plasma and water, is transferred from the blood to the dialysate due to osmotic, diffusive and convective action. Further, in a process known as ultrafiltration, excess fluid may be removed from the blood by establishing a pressure differential across the membrane that pulls the excess fluid from the blood across the membrane and combines it with the dialysate in the dialyzer. Dialysate discharged from the filtration device, sometimes referred to as spent dialysate, may be conducted past a heat exchanger where heat from the spent dialysate is transferred to the treated water being provided to the hemodialysis machine on the "fresh" side. Thereafter, the spent dialysate is conducted to a drain line for collection, analysis and/or, more typically, discharge.
A single hemodialysis machine generally does not run continuously, but rather is used to treat blood in discrete treatment sessions, usually with different patients. The equipment may be idle between treatments and may accumulate deposits in the flowpath. Further, spent dialysate may contain molecules or material which can accumulate in the flowpath after the dialyzer, which can provide a nutrient source for bacterial growth and accumulation therein. The use of such equipment for different patients, the need to prevent patient pyrogenic reactions due to bacterial endotoxin, and the possible accumulation of dirt or other unsterile substances in the equipment make periodic cleaning and disinfection of the equipment desirable.
Cleaning is typically achieved by rinsing the affected portions of the flowpath with bleach solution. Chemical and/or heat disinfection are methods commonly used to disinfect the non-disposable portions of hemodialysis equipment.
Chemical disinfection techniques include the conduction of chemicals such as formaldehyde, bleach, peracetic acid or other disinfectant solutions through the non-disposable portions of such equipment. There can be, however, significant cost associated with the purchase and use of such chemicals. In addition, chemical disinfection techniques require a technician to specially add, remove and/or dispose of the chemical disinfectant, while disconnecting the hemofiltration device and other components from the dialysate equipment. Performance of these steps can take time away from the technician's other duties. With certain disinfectants, the technician who is cleaning the system must take special care when dealing with concentrated chemical solutions. Moreover, in disinfecting with chemicals, care must be taken to completely flush the chemicals, which may exhibit some degree of toxicity, from the portion of the flowpath in which the dialysate is prepared and through which fresh dialysate flows during treatment, to avoid any possibility of delivering the chemical to the patient through the membrane of the dialyzer. There may also be environmental concerns or regulations which restrict the discharge of the disinfecting chemicals to public waste disposal facilities. Container disposal can also be problematical.
Heat disinfection of extracorporeal blood treatment systems is well known. Heat disinfection of the fluid pathway of extracorporeal blood treatment systems is performed by circulating a fluid such as water, sterile water, or a disinfection solution throughout all such pathways of the equipment for a sufficiently long period of time, typically 15 minutes or more, at a sufficiently high temperature, typically from 80.degree. C. to as high as 125.degree. C. One way in which such disinfection is achieved is by conducting the solution (1) through the pathway which ordinarily receives treated water, (2) into the portions of the flowpath in which fresh dialysate is prepared with heated water and in which fresh dialysate flows during treatment, (3) bypassing the dialyzer, (4) into the portions of the flowpath in which spent dialysate flows during treatment, and (5) through the drain line to exit the machine. This technique is referred to herein as a single path once-through heat disinfection method. Single path once-through heat disinfection discharges the heated fluid through the drain line to the drain. As a result, fluid continuously added to the system must be heated to temperature, which consumes additional power and results in additional cost.
This problem has been ameliorated in some hemodialysis systems by routing all or a portion of the heated fluid from the part of the flowpath in which spent dialysate flows during treatment back, to the part which receives treated water, or to the dialysate preparation portion, thereby conserving heat and reducing the power required to maintain adequate disinfection temperature. These techniques are referred to herein as single path heat disinfection methods with recirculation.
Although single path heat disinfection with recirculation reduces the power consumed by the disinfection process, it creates a new problem. The heated fluid, in passing through the spent dialysate line, may pick up material in the line deposited by spent dialysate during prior treatments. Such material may be subsequently carried into the dialysate preparation line and fresh dialysate lines. This creates a possibility of contamination which may potentially be passed on to patients subsequently treated with the equipment.
All single path heat disinfection methods may exhibit substantial heat loss between a point in the dialysate preparation line where the solution is heated and the drain line where solution is discharged from the system. This heat loss results in decreasing temperature from the dialysate preparation portion of the flowpath to the drain line which may result in incomplete disinfection of the flowpath approaching the drain line. The disinfection fluid in the dialysate preparation portion is sometimes heated to a temperature substantially above 90.degree. C., for example at or around 125.degree. C., so that the resulting fluid temperature gradient from the fluid in the dialysate preparation portion to the fluid in the drain line results in a low temperature of closer to 90.degree. C. However, this technique can result in damage to equipment which may not be designed for repeated operation at such elevated temperatures or can require the incorporation of material capable of withstanding elevated temperatures, thereby increasing system cost.
It is against this background that the significant improvements and advancements of the present invention have taken place in the field of disinfecting extracorporeal treatment apparatus.